The invention relates to a form-fitting cast-on enclosure arrangement having parallel chucking surfaces for the fixing of irregularly contoured metallic workpieces which are to be processed, as well as to a process for manufacturing and separating the enclosure arrangement.
An enclosure arrangement or fixture of the above-mentioned type which is made of metal is known from the Great Britain patent document GB-21 66 070, in which case a metallic high-temperature resistant workpiece with irregular contours, such as a power unit blade, is partially cast into a block of low-melting metal so that in the process parallel chucking surfaces are created and the further processing of the workpiece is therefore made possible. A disadvantage of this fixture is the fact that, during the cast-in operation, molten metal comes in contact with the workpiece surface so that there is at least the risk of contamination of the workpiece surface with foreign metals. In the high-temperature operation of the workpiece, an influencing of the stability characteristics in the area of the cast-on enclosure arrangement of the processed workpiece cannot be excluded. Another disadvantage is the high-expenditure chemical or chemical-physical cleaning of the surface of the workpiece when a contamination is to be removed. Disadvantageously, cracks may occur during the cooling of the molten bath of the enclosure arrangement material. Another disadvantage is the handling of the workpiece that is enclosed by metal because the cast-in block causes a considerable increase in weight. Finally, because of the softness of the low-melting metal with respect to the high-melting hard workpiece, a continuous cutting of the enclosure arrangement material and of the workpiece material is not possible.
In addition, the process known from the Great Britain patent document GB-21 66 070 is not economical, at least for repair work, because a metal melting energy must be applied and a complete metal casting process must be provided. This requires a correspondingly high-expenditure handling of molten baths and long cooling phases.
It is an object of the invention to provide a enclosure arrangement of the above-mentioned type and a process for its manufacturing and separation. In such enclosure arrangement, a common cutting processing of enclosure arrangement material and workpiece material is possible and contamination of enclosure material that does not endanger the workpiece stability is acceptable without any cleaning after treatment. At the same time, however, the contamination is negligibly low. In addition, the enclosure arrangement must be free of material cracks, and the parallel chucking surfaces must permit a secure fixing. It is also an object of the invention to provide a process which is so reasonable with respect to cost that used and damaged components can be fixed by the enclosure arrangement for the purpose of being repaired.
According to the present invention, the object is achieved by the fact that the enclosure arrangement consists of plastic material and has several spaced ribs which are connected by webs.
The enclosure arrangement according to the present invention has the advantage that, at high operating temperatures of the workpiece, plastic material burns in the oxidizing atmosphere so that individual molecule chains which may possibly still contaminate the metallic surface after the processing and separating step, will disintegrate and will not negatively change the stability of the workpiece. Plastic material can easily be handled because of its low specific weight. Using conventional machines, plastic material can be cast onto the workpiece in a few seconds, and the ribbed structure of the parallel chucking surfaces ensures a crack-free casting-on of the enclosure material because plastic material, when it cools from the softened condition, can emit sufficient heat by way of the ribs and webs in order to uniformly cool and shrink. In addition, at room temperature, plastic material still has a sufficient plastic material flowability in order to shrink free of cracks.
The preferred use of a thermoplastic material as the enclosure material results in special advantages. A thermoplastic material can be used several times because it is hard and tough at low temperatures and softens at higher temperatures. This permits an environmentally acceptable enclosure, because approximately up to 100% of the thermoplastic material can be reused without any cost-intensive processing steps.
A preferred material for the enclosure arrangement is polystyrene. This plastic material is hard and solid at room temperature and is capable of enclosing in ribs without any cracks an irregular workpiece contour after a casting-on at the softening temperature and cooling to room temperature. The stability is so high that a chucking of the ribs in conventional fixing tools is possible without any deformations. In addition, this plastic material can be processed in a cutting manner using the same water-cooled tools as the workpiece.
In the case of a first embodiment of the enclosure arrangement, the ribs and the webs have a cross-sectional surface of approximately the same size. This has the advantage that shrinking strains at the transitions points between the ribs and webs are minimized and a crack-free enclosure is permitted.
Another preferred embodiment of the process provides that, between the ribs, the workpiece is completely surrounded by the enclosure arrangement. As a result, on the one hand, the position of the ribs and the webs on the workpiece is additionally secured and, on the other hand, the workpiece can be metal-coated or processed electrochemically or chemical-physically in an advantageous manner on the exposed surfaces that are not covered.
A preferred application of the enclosure arrangement is for the fixing of power unit blades, in which case the webs at least partially enclose the leading edge and the trailing edge of the blade. This results in the advantage that the leading edge and the trailing edge are protected from mechanical damage during the processing operation and, at the same time, the position of the ribs is fixed on the blade.
The ribs are preferably arranged orthogonally or at an angle of between 45.degree. and 90.degree. with respect to the blade axis. The ribs preferably completely enclose the blade. This has the advantage that the blade axis can be aligned more precisely and a secure fixing of the blade device is ensured.
In the case of manufacturing and repair of a power unit blade, the blade is finished first. For this purpose, the blade is chucked in the area of the base or, when a cast-on shroud exists as an integral component, the blade is chucked on the shroud segment. For the finishing of the shroud segment and/or of the blade base, an enclosure is then cast around the blade in order to produce suitable parallel chucking surfaces. A preferred embodiment of the enclosure arrangement therefore provides that the distance between the ribs in the area of the blade base or the blade tip be smaller than in the blade center.
In the blade center, the distance between the ribs is preferably larger than twice the rib thickness. With this arrangement of the ribs, the chucking forces may advantageously be distributed on the blade without the occurrence of deforming chucking forces, particularly since blades in aircraft engines have extremely thin wall thicknesses in the blade center.
A preferred process for manufacturing an enclosure arrangement consists of fitting the workpiece into a receiving mold of an injection molding machine. The plastic material for the enclosure is then injection molded onto the workpiece. This has the advantage that, when polystyrene is used as the enclosure material, the areas to be processed, such as the blade base and the shroud segment, can also be cast in because a water-cooled cutting of both materials is advantageously possible using conventional tools. If the blade is completely surrounded by enclosure material, a cutting may advantageously be followed by a metal plating in the shroud area or in the blade area and/or electrochemical polishing or etching. Chemical-physical processing steps, such as vacuum metallizing or sputtering or chemical vapor depositing are also possible, in which case the finished blade surface is protected by a complete enclosure.
For separating the workpiece from the enclosure, the enclosed workpiece is cooled to a temperature at which continuous shrinking cracks are formed in the enclosure. Since, at low temperatures, the plastic material becomes brittle and shrinks more than the metallic workpiece, continuous shrinking cracks are formed in the plastic material without any deformation of the workpiece. Advantageously, the enclosure chips-off the workpiece without any residues and can be reused in the case of a thermoplastic material and can again be supplied to an injection molding process.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.